We investigate the fundamental problem of (terminating) distributed consensus in a challenging environment of large resource-limited mobile sensor networks. Namely, we consider the model of population protocols. In this model an unknown number of asynchronous, anonymous and nite state mobile agents interact in pairs. The agents are unable to detect which other agents are present but not interacting, even if no crash (halting) failures happen. This is the main reason for the impossibility of consensus in this model. After proving this impossibility result, we investigate the conditions to add to the original model for obtaining solutions. We adopt the known technique to encapsulate these conditions in an oracle, a distributed external module able to provide some helpful information (for solving problems). Some already known oracles do not t the model of population protocols, some others are not relevant to the consensus problem. We propose an entirely new category of oracles. We show how a specific oracle in this category allows to solve symmetric consensus, a stronger but natural version of consensus in the considered model. This solution tolerates any number of crash failures. Finally, we prove that the proposed oracle is the weakest in its category.
